The ball-and-seat check valve configuration is effectively an industry standard for implementing the inlet or outlet check valve functions in single-piston or multiple-piston pumps used in high-pressure liquid chromatography (“HPLC”) applications. A passively-actuated ball-and-seat check valve offers relative simplicity, good mechanical stiffness, chemical inertness, adequately-swept wetted geometry, high cycle life, and reasonable sealing performance when not perturbed by environmental factors. A common material configuration associates a ruby ball with a sapphire seat, although other material combinations, including a ceramic ball or a ceramic seat, are also commonly employed. Most ball-and-seat check valves used in chromatography pumps are oriented with the flow path directed vertically upward. In this orientation, the negative buoyancy of the immersed ball results in a mild downwardly-directed force which urges the ball toward the seat. Spring preloading of the ball against the seat may be used to augment the gravitationally-derived force acting to keep the valve in a normally-closed state. A passively-actuated ball-and-seat valve used as a pump outlet check valve can incorporate a moderate spring preload, acting in the reverse- or flow-checking direction, as under normal delivery conditions, liquid pressure derived from the pump can be used to overcome the spring force and open the valve, enabling liquid delivery to the receiving system. A heavier spring preload can be useful in contributing to reliable valve closure in the presence of entrained gas bubbles, flocculent precipitates, or relatively soft contaminants.
In a passive inlet check valve, only a very light spring preload force can be applied, as the force available for opening the valve is extremely small. During the liquid intake phase of pump operation, it is generally desirable to avoid the creation of a significantly sub-atmospheric pressure condition (i.e. a pressure significantly less than 1 bar absolute) within the pump cylinder and associated fluid conduits. A cylinder pressure condition during intake which departs significantly below atmospheric pressure may trigger the outgassing from solution of dissolved gases which may be present in the mobile phase, and a cylinder pressure approaching 0 bar absolute may trigger the generation of vapor pockets, or cavitation, within the liquid. In a conventionally-aspirated HPLC pump, where the solvent inlet path to the pump intake resides at substantially atmospheric pressure or 1 bar absolute, it is therefore normal practice to configure a passive inlet check valve to operate with a threshold cracking pressure differential in the forward- or flow-enabling direction which is on the order of 10000 Pascals. Due to the very small physical size of modern HPLC ball-and-seat check valves, the effective area over which the cracking pressure differential is asserted is correspondingly small, typically in the vicinity of 1.0×10−6 square meters. This configuration results in an available opening force which may be typically in the range of 0.01 to 0.02 N. This limitation on the available opening force for a passive inlet check valve results in a corresponding limitation on the closing force which can be selected for the valve.
With such small opening and closing forces available, the passive ball-and-seat inlet check valve is susceptible to failure, where the failure may originate from the valve becoming stuck in an open or a closed position. A stuck-open condition may arise when the ball is biased off the seat, and is thereby prevented from attaining the closed position, due to the presence of gas bubbles, flocculent precipitates, or other matter entrained in the mobile phase solvent stream as it transits the valve. A stuck-closed condition may arise from weak attraction or adhesion between the ball and the seat, due, for example, to electrostatic charging or to the presence of organic or inorganic residues or foreign matter in the solvent. In extreme cases, typically associated with improper system operation and storage, the ball and seat may become temporarily adhered together as a result of the precipitation and deposition of non-volatile buffer salts on the ball-seat assembly. Unlike the stuck-open conditions arising from gas bubbles or flocculent precipitate, which may be of a transient nature, an inlet check valve which is stuck in the closed position renders the pump inoperable until some mechanical intervention is made to free the valve. In an extreme exemplary condition where the precipitation, deposition, and eventual drying of non-volatile buffer salt residues on an inlet valve mechanism causes the valve to become substantially cemented closed, the assertion of an unusually large force may be required to achieve opening of the valve. However, the requirement for assertion of an unusually large force corresponds to an exceptional operating condition, which can be identified as distinct from the normal valve-opening and valve-closing conditions associated with ongoing pump delivery.
The positive-displacement pumps employed for delivery of HPLC mobile phases to a receiving system are subject to very stringent performance requirements, in that the chromatography mobile phase must be delivered with extremely high volumetric precision, at pressures which may range from approximately 7 MPa to approximately 140 MPa, or even higher. Reverse- or back-leakage of an inlet check valve degrades the performance of the pump by perturbing the relationship between piston displacement and the volumetric delivery of solvent to the system. It is therefore desirable to have the inlet check valve constructed so that in its closed state, the reverse-leakage rate, expressed as a volumetric flow rate, is on the order of nanoliters per minute or less. The high differential pressure which can exist across the inlet check valve in the reverse- or flow-checking direction, when the corresponding pump cylinder is pressurized to system pressure, or to a significant fraction of system pressure, places a significant requirement on the mechanical stiffness and general robustness of the valve. Lack of mechanical stiffness contributes to perturbing the relationship between piston displacement and volumetric delivery of solvent to the receiving system, particularly when the delivery pressure varies in response to a change in the load resistance represented by the receiving system. Change in the load resistance represented by the receiving system can occur during a chromatographic analysis as a result of a change in solvent viscosity due, for example, to the execution of a user-programmed time-varying solvent composition profile for sample elution.
One conventional configuration of an HPLC system utilizes a multi-channel solvent proportioning valve located upstream of the pump intake, such that a single high-pressure-capable HPLC pump can deliver user-configurable solvent mixtures or programmed time-varying solvent gradient compositions to a receiving system. This pump configuration is often referred to as a “low-pressure solvent gradient formation” or “low-pressure solvent gradient proportioning” system, to distinguish that configuration from a “high-pressure solvent gradient formation” system, wherein multiple high-pressure-capable pumps are used to directly generate solvent mixtures at the full system pressure. In a low-pressure solvent gradient formation system, a time-varying or a time-invariant solvent mixture is required to transit the pump. The solvent mixture formed by the interaction of the pump intake cycle with the proportioning valve cycling is susceptible to perturbation arising from non-ideal behavior of the inlet check valve. A known perturbation is the formation of an erroneous solvent composition as a result of error in the mapping or allocation of actual pump cylinder intake to one or more of the solvent channels selectable at the proportioning valve. Such errors in mapping may originate from irreproducible opening or closing behavior of the inlet check valve, or may originate from fluid volume displacement arising from motion of the valve actuator, as opposed to arising from known displacement of the pump piston. In addition, a poorly-swept pump inlet check valve can further degrade solvent gradient formation performance by contributing an undesirably large delay and mixing contribution to an intended time-varying solvent gradient profile as programmed by the user.